THE TREATMENT OF SEWAGE.

New Zealand Times, Volume XXX, Issue 4525, 21 September 1875, Page 3

THE TREATMENT OF SEWAGE.

(From The Times, May 21.)

On Wednesday afternoon the works of the General Sewage and Manure Company at Whitley, near Coventry, were thrown open by the directors for the inspection of a large number of gentlemen from various parts of the kingdom, who were invited to witness the methods employed by the company for the extraction of the valuable elements from the sewage of Coventry and for the purification of the water prior to its discharge into the River Sherburne.

The problem of how to deal with sewage to advantage can scarcely anywhere be presented under circumstances of greater difficulty than at Coventry, where the actual quantity delivered at the works amounts to two million gallons a day, and where the quality is liable to frequent variations from the intermittent flow of colored refuse from the different dyeing establishments in the town On this account, and in order to obtain a comparatively colorless effluent, it is necessary for the men in charge of the works to be constantly watchful, and to regulate the admixture of chymicals by the observation of the nature of the sewage as it arrives.

The works are situated on the banks of the little River Sherburne, about a mile and a half from the town of Coventry, and the sewage is brought to them by gravitation only, without the use of pumps. On its arrival it is first passed through one of Latham's solid sewago extractors, which, by means of an Archimedean screw, lifts out and delivers into a box all portions of an appreciable size. Among these some very miscellaneous articles are sometimes found, such as living fish and frogs, dead rats, fragments of pottery, spoons, and forks, pipes, bones, rags, and walking-sticks. When all obviously foreign matter has been removed, the residue, which amounts to about half-a-ton daily, is dried and used as agricultural manure.' Up to the present time the whole of this manure has been required for fruit trees and gardens on the grounds of the company. The liquid sewage, after passing through the extractor, flows beneath chambers, from which it receives the chymicals by which the precipitation of its dissolved material is effected. A shale from the neighboring coal and iron measures, which can be obtained in any quantity for the cost of cartage, is roughly broken, and then agitated with sulphuric acid in a proper receptacle. The crude sulphate of alumina thus produced is dried and powdered and is stirred with boiling water in a mixing tank so as to make a saturated solution, from which the superfluous sulphate is suffered to subside, and the supernatant liquid, while still hot, is made to flow into the sewage in such quantities as its condition may from time to time require. Passing on a little farther, the mixture receives an addition of milk of lime, with which it is well stirred, and it is. then allowed to flow into the subsidence tanks, which are four in number, and of which one is cleared out daily. Each tank is divided into two portions by a weir, and, as soon as the farther portion is filled, the effluent pours over another weir at its extremity into a channel which conducts it to a filtering bed of earth of 4J acres in superficial extent and sft. deep. After sinking through this filtering bed it finds its way into three mains, and is by them delivered into the Sherburne. The little river receives a certain amount of drainage from houses along its banks, and the effluent is far cleaner and better than the stream into which it enters. As dipped from one of the mains it has a little yellow color, but is perfectly free from any offensive characteristic. The company have recently set aside additional land for filtration, so as to have a second bed in reserve in case it should be necessary to suspend the employment of the first.

"When each tank is emptied every fourth day its bottom is found covered to the depth of an inch or more with the matters precipitated by and in combination with the sulphate of alumina and the lime. This precipitate is swept, in the state of mud, into an underground chamber below the level of the tanks, and there, after a further period of subsidence, and after its supernatant liquid has been suffered to flow back into the stream of fresh sewage, it is pumped up into a house, where, after being still further freed from moisture by an extremely ingenious revolving filter, it is dried over hot plates, and is delivered as a sort of earth, which has according to the analysis of Professor Voelcker, a manurial value of from 355. to 40s. per ton. A sewage manure of this description contains too much inert matter to be acceptable to farmers, and can only find a market after being sufficiently "fortified" to render it worth cartage. Quite recently, however, the company have hit upon a scheme which is not yet in full operation, but which was exhibited on a Bmall scale on Wednesday, and which, as soon as the necessary works for it can be constructed, will entirely supersede the present drying process. The wet precipitate, or sludge, as it is raised from the underground chamber, will be stirred with phosphate of lime and sulphuric acid, with the result that the water will be used up as water of crystallization for some of the salts produced, and that the whole mass will be rapidly dried by chymical action, without the employment of heat. This method will, it is said, effect a saving of £25 a week in fuel alone, and will at the same time " fortify " the sewage precipitate to such an extent as to yield a compound worth from ,£6 to £7 a ton for agricultural purposes. If this prospect should be realised and at the estimated cost, there can be no doubt that the works will yield a handsome profit to the proprietors, who will also deserve much gratitude for having accomplished what has so often been attempted in vain. It would of course be premature, from an operation conducted only xipon a laboratory scale, and prior to the development of a market for the product, to express any confident opinion with regard to the success or failure of this part of the undertaking. The question of national importance, however, is not so much how to turn sewage to profitable account, as how to prevent it from being injurious to health. The latter is an affair of necessity, the former is very desirable; but it cannot be put in the first rank. There can be no doubt that the works at Coventry, whatever may be the ultimate commercial value of their products, deal with the sewage nuisance in a perfectly satisfactory manner. The works themselves are models of order and good arrangement, which reflect the greatest credit upon Mr. Mellis, by whom they were designed and erected. The mechanical contrivance for the various processes arc exceedingly ingenious, the operations, from first to last, are entirely odorless and inoffensive, and the effluent water would be drank without question by anybody who was not aware of its source. In all these respects the example set by Coventry cannot be too closely followed elsewhere ; but it would probably be wise for local authorities to seek more than one method of rendering _ their sewage works profitable. If all towns in the kingdom were to manufacture artificial guano, the price of that commodity would bo likely to undergo a rapid decline, while if it were found possible to manufacture other things also, the overloading of tho market with a single product would be avoided.